1. Field of the Invention
The present invention relates to an antenna controller mounted on a movable body for controlling an antenna drive for driving a satellite tracking antenna.
2. Description of the Related Art
FIGS. 10 and 11 are block diagrams showing two examples of current structures in which a rate sensor is used for an antenna controller mounted on a movable body for satellite tracking. The rate sensors are attached to the movable body and are used to detect a turning angular velocity of the movable body. FIG. 12 is a characteristic chart showing characteristics of a rate sensor. By using the rate sensor, an output V is obtained for a turning angular velocity .omega.. A detected angular velocity .omega..sub.1 can be expressed by the following equation (1): EQU .omega..sub.1 =G.times.(V-V.sub.0) (1)
wherein .omega..sub.1 represents a detected angular velocity, G represents a factor, V represents an output of the rate sensor, and V.sub.0 represents an output in a stationary state (0.degree./S) (hereinafter referred to as a "reference output").
In FIGS. 10 and 11, the numeral 1 denotes an A/D converter for digitally converting an analog output voltage of the rate sensor, the numeral 2 denotes a lowpass filter for artificially calculating the output V.sub.0 of the rate sensor during stopping, the numeral 3 denotes a factor multiplier for converting a voltage (V-V.sub.0) to an angular velocity .omega., by multiplying (V-V.sub.0) by a factor of G, the numeral 4 denotes an integrator for calculating a turning angle .theta. from the detected angular velocity .omega..sub.1 converted by the factor multiplier 3, the numeral 5 denotes an antenna direction counter for detecting an angle at which the antenna is driven, the numeral 6 denotes a step tracking circuit for outputting a signal for moving the antenna right and left or back and forth in predetermined increments or steps and for detecting a direction of the antenna in which an electric wave sent from the satellite is intensified, and the numeral 7 denotes an antenna driving circuit for driving the antenna to decrease antenna direction errors.
In FIG. 10, the reference output V.sub.0 is artificially calculated by the lowpass filter 2 on the basis of the output V of the rate sensor which is digitally converted by the A/D converter 1. The factor multiplier 3 then multiplies (V-V.sub.0) by the factor G so that its output corresponds to the detected angular velocity .omega..sub.1 of the movable body. The detected angular velocity .omega..sub.1 is integrated by the integrator 4 so that the output of the integrator 4 corresponds to the turning angle .theta. of the movable body. An antenna direction error .theta..sub.err is calculated on the basis of the turning angle .theta., antenna direction information of the antenna direction counter 5, and a step width of the step tracking circuit 6. Thus, the antenna is driven such that the antenna direction error .theta..sub.err is reduced by the antenna driving circuit 7. Consequently, the antenna controller corrects the antenna direction with the turning angle .theta. and the antenna is continuously turned in a constant direction with respect to the ground. This enables satellite tracking.
FIG. 11 is the same as FIG. 10 except that the output V.sub.0 of the rate sensor in the stationary state is previously determined as a fixed value without use of a lowpass filter 2.
In the antenna controllers described above, tracking errors are common due to sensitivity errors of the rate sensor, installation angle errors of the rate sensor, differences in the artificial reference output obtained by the lowpass filter or a drift of the reference output, rolling caused when turning the movable body, and the like. Tracking error is alleviated by the step tracking circuit 6 which performs a closed loop control method using a received signal level, thereby enabling satellite tracking.
In a rate sensor that calculates turning angular velocity by subtracting the reference output V.sub.0 in the stationary state from the output V and then multiplying the resulting figure by the factor G, if the antenna controller calculates the reference output and then the output in the stationary state drifts, a difference will exist between the reference output recognized by the antenna controller and the true reference output which has drifted. For this reason, the calculated turning angular velocity includes greater errors. Consequently, when using an open loop control method for controlling the antenna with information on the turning angular velocity, tracking error actually increases over time until a satellite can no longer be tracked.
In order to decrease tracking error, the following method has been used. More specifically, an AC component is removed from the output of the rate sensor through a lowpass filter and the value thus obtained is artificially set to the reference output such that the reference output is frequently updated and drift is removed. However, when the movable body meanders or turns, difference between the artificially obtained reference output and the true reference output is increased. Under these conditions tracking error cannot be lessened.
A further method for decreasing the tracking errors is used by the step tracking circuit 6 which performs closed loop control method or the like. According to this method, if stability and band of a closed loop are restricted with respect to performance of a receiver or an antenna, an increase rate of the tracking errors caused by the drift of the reference output is sometimes greater than a decrease rate of the tracking error obtained by the closed loop control method. For this reason, this method is not practical for lessening tracking error.
For the above-mentioned reasons, the stability of the reference output of the rate sensor used for the open loop control method is important to the antenna controller according to the prior art. Therefore, the rate sensor whose reference output greatly fluctuates is difficult to use with a satellite tracking antenna controller mounted on a movable body. For this reason, rate sensors whose reference outputs do not fluctuate should be selected through stringent testing, or an optical fiber gyro or a like instrument with a stable reference output should be used. This raises the cost of the antenna controller.